SAN SALVADOR, El Salvador — Gunmen boarded a bus in a town near the Salvadoran capital and opened fired on passengers Friday, killing six, including three law enforcement officials, police said.

A police investigator and two guards who worked at a maximum-security prison were among the victims, officials said.

Another five people were wounded during the attack in the town of San Luis Talpa near San Salvador, said national police director Rigoberto Pleites.

Pleites said witnesses told police the assailants were gang members dressed in uniforms similar to those issued to state road maintenance workers. He said investigators haven’t confirmed the attackers belonged to a gang.

Onlookers wait as police investigators inspect the crime scene following an attack on passengers in a bus in the town of San Luis Talpa, south of San Salvador, on Friday. (JOSE CABEZAS/AFP/Getty Images

Authorities did not provide a motive for the attack. But police assistant director Mauricio Ramirez said there have been threats of increased violence in the Central American country in the coming days. He did not provide any other details.

In 2012, the Mara Salvatrucha and the rival Mara 18 street gangs agreed to a truce. Homicides dropped from 12 per day in 2011 to an average of 6.8 per day in 2013. But during the first three months of 2014, the homicide rate has spiked back up to about 8.9 killings per day.

Ramirez said officials have ordered that no police officers take time off in the coming days after at least 21 people were killed in the country within 24 hours.

Also on Friday, authorities found the bodies of six boat maintenance workers. The men’s bodies were found half-buried in a clandestine grave in the town of Jaquilisco and members of the MS-13 gang are suspected in the killings, police said.